$A$ gas mixture contains $25 \%$ He and $75 \% CH_4$ by volume at a given temperature and pressure. The percentage by mass of methane in the mixture is approximately . . . . . . (in $\%$)

Calculate the volume of $H_2$ gas kept at $STP$ if it contains as many $H$ atoms as in $98 \ g$ of $H_3PO_4$. (Atomic mass of $P = 31 \ u$) (in $L$)

$23 \ g$ of $Na$ will react with methyl alcohol to give:

$20.0 \ g$ of a magnesium carbonate sample decomposes on heating to give carbon dioxide and $6.0 \ g$ magnesium oxide. What will be the percentage purity of magnesium carbonate in the sample? (Atomic weight of $Mg = 24$)

$A$ flask is weighed when filled with methane $(CH_4)$. The methane is then removed,and the flask is weighed again. When the flask is filled with oxygen $(O_2)$ at the same temperature and pressure,the mass of oxygen will be:

What is the weight of oxygen required for the complete combustion of $2.8 \, kg$ of ethylene? (in $kg$)